sql/mysql_priv.h:
  Changed last_query_cost status variable from global to thread-local.
sql/mysqld.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_class.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_class.h:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_select.cc:
  Changed last_query_cost status variable from global to thread-local.
sql/sql_show.cc:
  All SHOW_DOUBLE-type status variables are now assumed to be thread-local (there is 
  only one currently, last_query_cost, and this cset makes it thread-local)